A light emitting diode generally refers to a PN junction diode in which a p-type semiconductor and an n-type semiconductor are coupled.
In the light emitting diode (LED), when voltage is applied to the p-type and n-type semiconductors coupled to each other, holes of the p-type semiconductor are migrated toward the n-type semiconductor and electrons of the n-type semiconductor are migrated toward the p-type semiconductor such that the electrons and the holes are migrated into a PN junction.
The electrons moved into the PN junction are coupled to the holes while dropping from the conduction band to the valence band. Then, energy is released in the form of light according to an energy gap between the conduction band and the valence band.
Such a light emitting diode is a semiconductor device capable of emitting light and has various advantages such as eco-friendliness, low voltage, long lifetime, low price, and the like. Although such a light emitting diode has been typically used in an indicating lamp or in displaying numerals or other simple information, the light emitting diode has recently been used in many different applications, such as displays, headlamps of vehicles, projectors, and the like, with the development of industrial technologies, in particular, information display technologies and semiconductor technologies.
However, since it is difficult to fabricate a homogenous substrate for growing a semiconductor layer of the light emitting diode, the semiconductor layer is grown on a growth substrate having a similar crystal structure by metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), and the like.
As the growth substrate, a sapphire substrate having a hexagonal system is generally used. However, sapphire is a nonconductor of electricity and limits a structure of a light emitting diode to be formed thereon.
Therefore, various studies have been made to develop a technique for fabricating a vertical type light emitting diode by growing an epitaxial layer for the semiconductor layer on a sapphire substrate or a similar growth substrate, and separating the growth substrate.
To separate the growth substrate, the substrate may be removed by grounding. However, the grounding technique for removing the growth substrate, that is, the sapphire substrate, is associated with long processing time and high cost.
Therefore, a laser lift-off (LLO) method, a stress lift-off (SLO) method, or a chemical lift-off (CLO) method is generally used to separate the epitaxial layer from the growth substrate.
In the LLO method, an epitaxial layer is grown on a growth substrate, a bonding substrate is bonded to the epitaxial layer, and a laser beam is emitted through the sapphire substrate so as to separate the epitaxial layer from the growth substrate.
In the SLO method, a convex-concave pattern is formed on one surface of a growth substrate, only a partial region of the growth substrate is subjected to passivation with an insulation film such that an epitaxial layer can be grown only on the other region of the growth substrate, and the epitaxial layer is thickly grown and then cooled, thereby allowing the epitaxial layer to be separated by surface stress.
In the CLO method, a chemically vulnerable material is deposited in a predetermined pattern on one surface of a growth substrate, and then an epitaxial layer is grown and separated by electrochemically or chemically removing the chemically vulnerable material from the growth substrate.
Among these methods of separating the growth substrate, the LLO method has a problem in that properties of the epitaxial layer can be deteriorated by heat generated from a laser beam. Further, the SLO method or the CLO method has a problem that a process becomes complicated since a separate process for processing the surface of the growth substrate is performed before growing the epitaxial layer. In addition, the SLO method or the CLO method has a problem in mass production since it takes more time to separate the epitaxial layer in practice. Moreover, the application of the SLO method is not easy since the epitaxial layer must be grown thickly enough to be separated.